Field of the Invention
The present invention relates to a ring rolling process, a ring rolling apparatus and products obtained or obtainable using the ring rolling process and/or apparatus. Ring rolling is a category of materials forming, in particular metal forming.
Related Art
Ring rolling is a bulk metal forming process that typically generates large (1-5 m diameter, for example) metal rings for engineering applications such as aerospace, energy conversion and oil and gas extraction industries.
A metal workpiece in the shape of a ring having a starting outer diameter is rolled into a seamless ring of diameter larger than the starting diameter. Considering a ring-shaped workpiece having an axisymmetric shape and a rectangular cross section, the surfaces of the workpiece can be defined as radial surfaces and axial surfaces. An inner radial surface is located at the inner circumference of the workpiece and an outer radial surface at the outer circumference of the workpiece, each coaxial with the principal axis of the workpiece and orthogonal to the radial direction of the workpiece. First and second axial surfaces (e.g. upper and lower axial surfaces) are parallel to the radial direction of the workpiece and orthogonal to the principal axis of the workpiece.
Ring rolling processes known as radial ring rolling processes use two rolls, a forming roll (typically driven) acting on the outer radial surface of the workpiece and a mandrel roll (typically idle) acting on the inner radial surface of the workpiece. The ring is progressively reduced in cross sectional area, resulting in a corresponding increase in the diameter of the ring.
As a modification of radial ring rolling, radial-axial ring rolling processes are known which add axial rolls diametrically opposite the forming and mandrel rolls, i.e. on the other side of the ring. A typical arrangement for radial-axial ring rolling is shown in FIGS. 1A and 1B. Workpiece 10 is progressively formed between forming roll 12 and mandrel roll 16, acting on outer radial surface 14 and inner radial surface 18 respectively. Two guide rolls 20, 22 bear against the outer radial surface 14 of the workpiece to centre and stabilize the workpiece. At a position which is 180° angularly displaced around the principal axis A of the ring from position 24 of the roll bite between the forming roll 12 and mandrel roll 14, lower axial roll 26 and upper axial roll 28 bear against the first 30 and second 32 axial surfaces of the workpiece 10, in order to control the axial height of the workpiece as it is formed.
Han et al [Reference 11] disclosed the possibility of a ring rolling process in which the diameter and thickness of the workpiece are reduced during the forming process (as in the process described with respect to FIGS. 1A and 1B) but also the height of the workpiece (the axial extent of the workpiece along the principal axis direction) is increased. Intervening between the forming roll and the workpiece is a constraint cylinder. As the workpiece is progressively deformed, the axial height and diameter of the workpiece increase, the limit of the outer diameter of the workpiece corresponding to the inner diameter of the constraint cylinder, but the growth in the axial height not being constrained.
The discussion above is restricted to the formation of rings of rectangular cross section. It is also known to be of interest to form rings of more complex cross section. This is of particular interest where the desired end product has a relatively complex cross section. One approach to form such shapes is to form a rectangular cross section ring and then machine it to shape. However, this results in a low yield process, in the sense that much of the material of the original workpiece is removed. Furthermore, some benefits of ring rolling (in particular the generation of fine and/or textured microstructures near the surface of the workpiece) may be lost, at least in part. It is to be noted that other benefits of ring rolling are typically improved process speed compared to forging and improved microstructure compared to casting.
In principle a ring of complex cross sectional shape can be achieved using a shaped mandrel roll, shaped forming roll, or both, in order to form a near net shape product. However, a drawback of this approach is that different desired cross sectional shapes require the use of different forming tools, meaning that low volume production of complex cross sectional shapes by ring rolling is not cost effective.
FR-A-2040361 discloses a ring rolling process in which a forming roll, mandrel roll and first and second axial rolls are displaceable along their axes of rotation in order to accommodate a reduction in cross sectional area of the workpiece during ring rolling, in a configuration similar to that shown in FIG. 14 of this disclosure and discussed in more detail below. As such, the disclosure of FR-A-2040361 is limited to the production of rectangular cross sectional shapes only. Separately, FR-A-2040361 also discloses a ring rolling process in which the forming roll has a particular shape which is imparted to the workpiece in order to generate a non-rectangular cross section. It is clear from the disclosure of FR-A-2040361 that the shaped forming roll is not displaceable along its axis of rotation relative to the workpiece, and thus the cross sectional shape achievable is strictly limited to the cross sectional shape corresponding to the outer surface of the forming roll. The shaped forming roll is also not independently axially positionable relative to the mandrel roll in FR-A-2040361.
Tiedemann et al [Reference 5] disclose an approach in which one forming tool (in this case a mandrel roll) can be used to generate different cross sectional shapes in the workpiece by control over the axial and radial movement of that forming tool. The approach of Tiedemann et al is illustrated in FIG. 2 (taken from Reference 5) in which the forming roll 40 and guide rolls 42, 44 are located as in FIGS. 1A and 1B. Mandrel roll 46 has an annular projection 48. The mandrel roll is capable of radial movement but also capable of axial movement. This has the result of forming different profile shapes for the inner radial surface of the workpiece 50. It should be noted, however, that Tiedemann et al have not demonstrated control of the movement of the mandrel roll resulting in a required workpiece shape. Rather, Tiedemann et al have considered the resultant workpiece shape based on a predetermined movement of the mandrel roll.